Titanium and ferrous metals cannot easily be joined directly to each other by welding, because the resulting titanium/ferrous metal mixture, or alloy, is extremely brittle and usually will not survive the shrinkage stresses imposed upon it during cooling from the welding temperature.
To weld titanium to ferrous metal, an intermediate metal, which is compatible with both the ferrous metal and the titanium, is positioned between, and welded to, each of the two metals. The intermediate metal forms an alloy with the titanium and with the ferrous metal, thus joining the titanium to the ferrous metal.
Among the metals known to be compatible with titanium and ferrous metal is vanadium. An excellent discussion concerning the welding of ferrous metal to titanium using vanadium can be found in "The Welding of Titanium to Steel", Welding Journal (December 1961), D. R. Mitchell and H. D. Kessler.
Titanium is commonly used to line various containers. However, since titanium is much more expensive than various other materials, such as steel or other ferrous metals, the titanium liners are made as thin as is absolutely necessary to conserve material. Frequently, such thin layers of titanium develop leaks much faster and easier than would a thicker titanium. This problem is especially severe at points where titanium has been welded to a ferrous material through a vanadium intermediate due to alloying of the thin layer of titanium during welding, leaving little sound protective material against corrosive environment.
One reasonable means to follow to weld titanium to a ferrous material is to use a thicker sheet of titanium. This, however, is wasteful because the extra thickness is needed only at the point of welding, and is not needed throughout the entire body. Additionally, if the titanium sheet is to be formed into shapes, for example elongated ribs or vanes, it is very difficult to form thicker sheets of titanium, without cracking the titanium. In addition, thicker sheets of titanium require higher welding current and longer welding times and, therefore, generate more heat. The extra heat tends to crack or deform the titanium sheet. It has also been found that thicker titanium sheets do not bond well.
It would be highly desirable to have a method to weld a very thin sheet of titanium to a ferrous material using an intermediate metal, yet avoiding the problem with leaks caused by the weld.
The present invention provides such a method.